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FLUID Mechanics

Prepared by:

Brosk F. A. Zangana

Lecture no. 5

Department of Petroleum Engineering

Faculty of Engineering

Koya University

Email: brosk.zangana@koyauniversity.org

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Measurement of Pressure

The pressure on a fluid is measured in two different systems. It is

measured above the absolute zero or complete vacuum and it is

called the absolute pressure and in other system, pressure is

measured above the atmospheric pressure and it is called gauge

pressure. Thus1:

1- Absolute pressure: defined as the pressure which is measured with

reference to absolute vacuum pressure.

2- Gauge pressure: defined as the pressure which is measured with the

help of a pressure measuring instrument, in which the atmospheric

pressure is taken as datum. The atmospheric pressure on the scale is

marked as zero.

3- vacuum pressure: is defined as the pressure below the atmospheric

pressure.

1 A Text book of Fluid Mechanics and Hydraulic Machines by Dr. R.K.Bansal

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Measurement of Pressure

Figure 6.1

Absolute pressure= Atmospheric pressure+ Gauge pressure

Vacuum pressure= Atmospheric pressure – Absolute pressure

• The atmospheric pressure at sea level , 15 C is 101.3 kPa.

• The atmospheric pressure head is 760 mm of mercury or 10.33 m of water.

Tutorial

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Pressure Measuring Devices

The pressure of a fluid is measured by:

1- Manometers 2- Mechanical Gauges

Manometers: defined as the devices used for measuring the

pressure at a point in a fluid by balancing the column of fluid by

the same or another column of the fluid.

Mechanical Gauges: defined as the devices used for measuring

the pressure by balancing the fluid column by the spring or dead

weight.

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Manometers

Manometers are classified as:

1- Simple Manometers 2- Differential Manometers

A simple manometer consists of a glass tube having one of its

ends connected to a point where pressure is to be measured

and other end remains open to atmosphere.

Piezometer U- tube manometer Single column manometer

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Piezometer

Piezometer tube is perhaps the simplest of the pressure

measuring devices and consists of a vertical tube. In its

application one end is connected to the pressure to be

measured while the other end is open to the atmosphere.

The gauge pressure at A,

PgA = pxgxh

The absolute pressure at A is

PAab = pxgxh + Patm

This method can only be used for liquids (i.e. not for gases).

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U-tube manometer

Gauge pressure Vacuum pressure

If B is the point at which pressure is to be measured, whose value is P.

The datum line is A-A:

Let,

h1 = Height of light liquid above the datum line.

h2 = Height of heavy liquid above the datum line

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U-tube manometer

Gauge Pressure:

Pressure above A-A line in the left column = P+p1 x g x h1

Pressure above A-A line in the right column = p2 x g x h2

By equating the two pressures P+p1 x g x h1= p2 x g x h2

P = (p2 x g x h2 - p1 x g x h1)

Vacuum Pressure:

Pressure above A-A line in the left column = p2 x g x h2 + p1 x g x h1+ P

Pressure above A-A line in the right column = 0

By equating the two pressures p2 x g x h2 + p1 x g x h1+ P=0

P = - (p2 x g x h2 + p1 x g x h1)

Tutorial

Tutorial part 1

Part 2